Spatial Structure in End-Gas Autoignition

Numerical investigations are reported on the location of sites at which autoignition first develops in the end-gas ahead of a spark-ignited flame in a combustion chamber following rapid compression of an alkane + air mixture to high pressures and temperatures. Attention is drawn to the part played by the reactions that give rise to a negative temperature coefficient of reaction rate in an inhomogeneous temperature field. A ‘reduced’ kinetic mechanism was employed to model the spontaneous oxidation of n-alkanes. Flame propagation was described in terms of the ‘eddy dissipation concept’ and coupled to the more detailed mechanism by means of a switching algorithm. The CFD calculations were performed by use of KIVA II.

Two ‘knock related’ patterns were resolved as (i) intense pressure waves at high compressed gas temperatures which were initiated from autoignition ‘centres’ in the ‘core’ of the end-gas, and (ii) a relatively benign pressure wave development when autoignition was initiated in the vicinity of the piston crown. Autoignition ‘centres’ that occurred close to the piston crown and chamber wall were initiated at compressed gas temperatures which corresponded to those of the “negative temperature dependent” region.

Experiments on stoichiometric mixtures of n-heptane were performed in a rapid compression machine, and the initial and boundary conditions for the calculations were linked to those of the apparatus. Results are reported which give some support to the numerical investigations.